Make the loading scene open in the current scene tab if the current scene is empty

[Rindbee]

Previously, even if the scene in the current scene tab was empty (no scene root added and no scene path assigned), a new scene might be opened in a new tab when opening the scene through EditorNode::load_scene().

b2f425f	This PR

This is even worse when dragging the scene file to the 2D screen.

#90559	#90559 + This PR

[akien-mga]

What if the "empty" scene is actually not empty but just unsaved? I.e. if you start adding some nodes to it, don't save, and then open a new scene? I wouldn't expect it to discard the unsaved work.

[Rindbee]

What if the "empty" scene is actually not empty but just unsaved? I.e. if you start adding some nodes to it, don't save, and then open a new scene? I wouldn't expect it to discard the unsaved work.

In this case, the scene root is not null, the scene is unsaved not empty, so a new tab will be opened.

[KoBeWi]

When you delete scene root and load another scene, it will replace the "empty" one and will be marked as modified. Interestingly, you can undo that to restore the old root and it doesn't break anything 🤔
A scene is truly empty when it has no root node nor scene path assigned (rootless scenes can still be tied to a file if you deleted the root node).

[Rindbee]

0.mp4

It should be fine now, but dragging the scene file into the 2D Screen causes the shortcut Ctrl + Z to temporarily not work.

[KoBeWi]

I can't reproduce the issue in your video. Seems like the node was added to wrong scene tab?

nevermind, found another corner case :D

[KoBeWi]

Another bug:

1. Create new empty scene
2. Add root
3. Delete root
4. Open scene

The undo history is carried to the new scene. You should clear it.

[akien-mga]

Thanks!